They are capable of detecting it because they aren't putting much effort into being undetectable. If there was a need, uBlock Origin itself could be made entirely undetectable.

Of course the YouTube script running in your browser will be able to detect changes made to the page and request blocking. However, the said script can be modified by a different extension to either receive incorrect data about blocked requests and page information, or to send a fabricated result back to the server. Google can react to it by modifying the script, and the extension would need to adapt accordingly. It's a game of cat and mouse.

If there was a need, we could have YouTube running in an entirely clean headless browser with no adblockers, while the real browser we use pulls data from it and strips out the ads.

Ultimately, currently we have the last word on what happens on our end. Unfortunately, Google's webDRM, pushed by traitors to humanity Ben Wiser, Borbala Benko, Philipp Pfeiffenberge and Sergey Kataev, is trying to change that.